This invention relates to a method for preventing sedimentation of finely powdered coal in a colloidal fuel.
The colloidal fuel is a fuel obtained by having finely powdered coal of a particle size of the order of few microns dispersed in heavy oil. Since the specific gravity of the finely powdered coal is about 1.5 and that of heavy oil is about 0.95, the finely powdered coal gradually settles in the heavy oil with lapse of time by reason of the particle diameter of the dispersed particles (finely powdered carbon), the viscosity of the dispersion medium (heavy oil) and the difference in specific gravity.
With a view to preventing this phenomenon of sedimentation of the powdered coal, there has been proposed a method which involves addition of a surface active agent to the colloidal fuel. Even by this method, the phenomenon of the sedimentation cannot completely be avoided when the colloidal fuel is stored for a long time. There has also been proposed a method which prevents the sedimentation of finely powdered coal by keeping the colloidal fuel in a storage tank in an agitated state by means of a stirrer provided in the bottom portion of the storage tank. For the operation of the stirrer, there must be installed a pump of special design. Otherwise, heavy friction occurs between the pump and the movable mechanical parts of the stirrer which are exposed to direct contact with the colloidal fuel and accelerates wear of the pump and the stirrer.
In view of the general rule that the sedimentation velocity of dispersed particles is inversely proportional to the viscosity of a dispersion medium, it is readily inferred that the sedimentation velocity of the finely powdered coal can be lowered by increasing the viscosity of heavy oil. The simplest way of increasing the viscosity of heavy oil is by lowering the temperature at which the colloidal fuel is stored. This method of increasing the viscosity of heavy oil by lowering its temperature proves to be hardly feasible because of the great cost required for the cooling or heating treatment.
An object of this invention is to provide a method for preventing the sedimentation of finely powdered coal by quickly increasing the viscosity of the colloidal fuel to a specified value and thereby lowering notably the sedimentation velocity of the finely powdered coal.